Fuel cells, which generate electricity by electrochemical reaction between a fuel gas and an oxidant gas, are drawing attention as an energy source. Among such fuel cells, a solid polymer type fuel cell has a membrane-electrode assembly (hereinafter, also referred to as “MEA”) in which an anode electrode catalyst layer and a cathode electrode catalyst layer are formed on two sides of a polymer electrolyte membrane (hereinafter, also referred to simply as “electrolyte membrane”) that has proton conductivity. Gas diffusion layers made up of a carbon cloth, a carbon paper and the like are stacked on the two sides of the MEA. The gas diffusion layers and the MEA are joined by thermocompression bonding or the like in a production process of the fuel cell.
The electrolyte membrane provided in the solid polymer type fuel cell exhibits high proton conductivity when in a wet state, and the proton conductivity thereof declines with declines in the water content thereof. A decline in the proton conductivity leads to a decline in the electricity generation performance of the fuel cell. In order to maintain an electric cell performance of the fuel cell, it is necessary to maintain a sufficient water content in the electrolyte membrane. However, in general, fuel cells are often used outdoors. Therefore, particularly when the temperature is high, the water having evaporated in the MEA (i.e., the electrolyte membrane and the electrode catalyst layers) passes through the gas diffusion layers, and is let out of the fuel cell together with a reactant gas, so that the water content of the MEA declines. As a result, there arises a problem in which the proton conductivity of the electrolyte membrane sharply declines, and therefore the performance of the fuel cell declines. WO 2007/029346 discloses an organic-inorganic hybrid material that is capable of providing high proton conductivity in a wide temperature range from low to high. However, as for the gas diffusion layers stacked on both sides of the MEA, sufficient contrivance has not been made.
The foregoing problem is common not only to fuel cells that have gas diffusion layers described above, but also to fuel cells that do not have gas diffusion layers (e.g., a fuel cell that is not equipped with gas diffusion layers but employs separators in which a gas channel is formed in a surface that contacts the membrane-electrode assembly).